Most IC devices are assembled using a lead frame. Lead frames may be manufactured in long strips of many individual units. The long strips may be wide enough to accommodate from one to more than five lead frames. When a strip is more than one lead frame wide, the strip is referred to as a matrix. A lead frame may be equipped with carrier rails and guide holes to position the lead flames during manufacture. Each lead frame may be comprised of a plurality of leads having internal and external leads, tie bars, and a die pad. The die pad is centrally located on the lead frame and provides a surface on which a chip may be mounted. Lead flames are commonly of various sizes dependent upon the size of the chip, e.g., the semiconductor chip, and the number of connections that are made to the chip.
Conventional lead frame plating processes produce lead flames with clean, non-reactive finishes. The die pad of such lead frames is downset during packaging and a chip is mounted to the die pad. Connection areas on the chip, e.g., bond pads, are connected to the lead frame's internal leads by wire bonds. The die pad, chip, leads, and associated connections are encapsulated in an encapsulating material, e.g., a plastic mold compound, and excess lead frame material, such as the carrier rails, may then be trimmed away.
The leads are spaced around and connected to the chip by bond wires. Moreover, because of the many sizes of chips, there are currently many different sizes and configurations of lead frames, such as quad flat pack or dual-in-line configurations. Each of these lead flames is manufactured separately by lead frame suppliers and IC device manufacturers. Complicated lead frames configurations may be made by expensive stamping tools and dies or by chemical etching. Further, preparation of stamping equipment or etching processes for manufacturing lead frames may result in increased lead frame costs and extended cycle time from demand through development to production. These costs and delays may be reincurred with each new lead frame design.
A source of failure in IC device packages is the delamination of the encapsulating material from the lead frame. This may result from various causes including improper curing of the encapsulating material, surface contamination of the lead frame, and "popcorn failure." Popcorn failure may occur when liquids, such as water, are trapped between the encapsulating material and the die pad or when liquids seep or vapors condense under the encapsulating material. Moreover, plastic mold compounds are naturally capable of absorbing liquids from their environment until saturation occurs. Such liquids may be vaporized during IC device operation, and the expanding vapor, e.g., steam, may cause pressure to build up between the plastic mold compound and the die pad. This pressure build up may cause a catastrophic molded package failure in the form of a crack or delamination.
Encapsulating material failures may also result from thermal mismatch between dissimilar device materials, such as between the lead frame and the encapsulating material, at solder reflow temperatures, e.g., temperatures in a range of about 419.degree. to 464.degree. F. (215.degree. to 240.degree. C.). Such mismatches may be aggravated by the pressure created by the heating of liquids absorbed by the plastic mold compound. The combined effect of these stresses degrades adhesion, leads to delamination--especially between the lead frame and the encapsulating material, and may cause encapsulation cracking or failure.
Efforts to resolve these problems have involved choosing and developing encapsulating materials that better adhere to the chip or the die pad, or both. Encapsulating materials that are lower stress, stronger, absorb less moisture, and possess better adhesion capabilities have been proposed. Nevertheless, these have not eliminated the problems. Further, because specialized encapsulating materials are generally more expensive, their use tends to increase the costs of manufacturing. Further, additional process steps, such as dry packing of the finished device to reduce exposure to contaminating liquids, such as water, or texture etching of the die pad, have been added to manufacturing processes. Nevertheless, these also have increased lead frame manufacturing costs.